1. Field of the Invention
The present invention relates to a data communication system for communicating data such as a image data or a character data.
2. Brief Description of the Prior Art
In a prior art data communication system such as a G3 facsimile machine which meets the CCITT Recommendation, in a so-called fall-back mode a plurality of transmission rates are tested in a descending rate order in accordance with the status of transmission line in a pretransmission step to determine an appropriate transmission line to be used.
In this test or training, a training signal for converging an auto-equalizer of a modem in a receiver and a TCF (training check) signal are transmitted in a pair. The TCF signal is a modulation signal which has a data value "0" for 1.5.+-.0.15 seconds. In the prior art system, the training is checked by checking that there is no data error for at least a predetermined period (e.g. 1 second) after the receipt of the TCF signal or the number of data errors in an entire training check period is smaller than a predetermined number.
However, since the equalizer of the modem in the receiver converges with the time, there is a high possibility that a training failure may be detected even if the equalizer converges in the latter half period of the TCF signal and fall-back to a lower transmission rate takes place.